Sputter deposition, also known as sputter coating, is a technique for depositing thin films of a desired material on a substrate such as, for example, a magnetic disk for a hard disk drive or a semiconductor wafer. In general, ions of an inert gas from a plasma are accelerated toward a target of the material to be deposited. Free atoms of the target material are expelled when the ions collide with the target. A portion of the free atoms are collected on the surface of the substrate and form a thin film.
One well-known sputtering technique is magnetron sputtering. Magnetron sputtering uses a magnetic field to enhance the sputtering yield. Magnets are positioned behind the target, and magnetic field lines penetrate the target and form arcs over its surface. The magnetic field helps to confine electrons in an area near the surface of the target. The resulting increased concentration of electrons produces a high density of ions and enhances the efficiency of the sputtering process.
In a typical disk sputtering system, a disk is placed between two sputter sources, and thin films are deposited on both surfaces of the disk simultaneously. An example of a sputtering system of this type is the Model MDP-250B, manufactured and sold by Intevac, Inc. Another type of two-sided sputtering system is disclosed in U.S. Pat. No. 4,183,797 issued Jan. 15, 1980 to Kennedy et al.
A critical step in the manufacture of hard disk drive (HDD) media is the sputtering of thin films onto the media (disks). In order to achieve high recording densities, high coercivity and low noise media are required. It is also increasingly important that these properties remain uniform across the surface of the disk and from one disk to another. With the increased use of magneto-resistive heads, more stringent uniformity requirements are being placed on the media.
HDD media typically have several films, including underlayers, a magnetic recording layer and a protective overcoat layer. These thin films may all be deposited in the same sputtering system. Any of the layers can be deposited with the substrate at a bias potential using a technique commonly known as "biased sputtering". This technique differs from the basic sputtering process described above in that a voltage is applied to the disk. The voltage attracts to the disk ions which bombard the film that is being deposited and modify its properties.
Sputtering systems are usually designed for optimal film uniformity, target lifetime and collection of sputtered material (for increased cleanliness and reduced particle generation). Such design features are not always consistent with producing uniform films during biased sputtering, because the properties of the films produced using a bias on the disk also depend on the energy, angle and number of energetic ions bombarding the disk. These variables depend sensitively on the location and shape of electrodes within the system. In this context, electrodes are components of the sputtering system that have voltages applied to them, such as the target and the disk, or grounded conductors, such as chamber walls and shields.
Two sets of electrodes have primary influence on the properties of biased sputtered films. First, the disk is an electrode having a bias voltage that is virtually constant across its surface. This voltage drops immediately to zero, or to the voltage of the plasma in the chamber, beyond the edge of the disk. Since the disk attracts ions during biased sputtering, the shape of the disk and the applied voltage have a strong influence on the spatial distribution of the ions that bombard the disk. Typically, the voltage is applied to the disk through a substrate holder. Because the substrate holder is optimized for disk handling rather than for its impact on biased sputtering, it is usually non-uniform in shape. Non-uniform substrate holders induce localized electric field variations that produce images in the deposited materials (e.g., electrical, optical and mechanical images). This results in non-uniform biased sputtering. Second, the shields that are used to collect sputtered material and which serve as grounded surfaces for the gas discharge also influence the trajectories of the ions bombarding the disk. The shape of the shields may significantly alter the angles and the number of ions bombarding different locations on the disk. Thus, the shields may produce non-uniformities in the properties of the biased sputtered films.
Accordingly, there is a need for improved sputtering methods and apparatus wherein the above described non-uniformities are reduced or eliminated.